An electrophotographic apparatus of a transfer type will be described as an example. In the electrophotographic apparatus in which a toner is electrically charged to form an image by using an electrostatic force, the charged toner is supplied from the developing device to the electrostatic latent image formed on the photosensitive member as the latent image bearing member so that the electrostatic latent image is visualized (developed) as the toner image. The toner image is transferred onto a recording material (paper or the like) and is fixed as a fixed image on the recording material under heat and pressure by a fixing device, and then the recording material is outputted as an image-formed product.
A density of the image to be outputted is determined by a toner amount per unit area on the recording material and a toner characteristic after the fixing. The present invention relates to the developing device for supplying the toner in a desired amount to the electrostatic latent image and therefore the amount of the toner used for development will be briefly described below. In the case where the electrostatic latent image is the same, the amount of the toner used for development is smaller with a higher charge amount of the toner, and on the other hand, is larger with a lower charge amount of the toner. Thus, the density of the output image is changed depending on the toner charge amount and therefore in order to output the image with a stable density, there is a need to properly keep the toner charge amount.
In the electrophotography, the toner is controlled by using the electrostatic force even in other steps, and therefore it is important to keep the toner charge amount at a desired value. However, the present invention relates to the developing device and therefore in the following, the developing device will be principally described.
The developing device includes, in an ordinary case, a developing container in which a screw for circulating the developer therein while stirring and feeding the developer is incorporated, and includes a developer carrying member, which opposes the latent image bearing member, for conveying the developer toward the latent image bearing member. Further, in the ordinary case, in order that the amount of the developer which is carried on the developer carrying member and which is conveyed toward the neighborhood of the latent image bearing member is made a desired value, a developer regulating member for regulating a gap (spacing) between the developing container and the developer carrying member is provided in the developing container side.
In the present invention, as the developer, the two-component developer comprising a non-magnetic toner and a magnetic carrier is used. The two-component developer may contain no magnetic material in the toner and, therefore for the reason that a color (tint) is good or the like reason, the two-component developer has been widely used in a color image forming apparatus.
In the case of the developing device using the two-component developer, in general, as the developer carrying member, a developing sleeve in which a magnet roller (magnetic field generating means) having a developer carrying ability is incorporated is used. The developing sleeve carries the developer by a magnetic force of the magnet roller and is rotated to convey the developer to the latent image bearing member, thus supplying the toner to the electrostatic latent image. The toner is electrically charged by friction with the carrier and therefore the toner charge amount is different depending on a ratio between the toner and the carrier. The toner and the carrier in the developer are charged to opposite polarities and the sum of the charge amounts is zero, and therefore the toner charge amount becomes smaller with a higher toner ratio in the developer.
That is, even with respect to the same electrostatic latent image, the toner charge amount is changed by a change in toner ratio and therefore the amount of the toner used for development is different. For this reason, in the ordinary case, the toner in substantially the same amount as that of the toner consumed by image formation is supplied so as to control the toner charge amount at a substantially constant level, so that the ratio between the toner and the carrier is used within a certain range. When the toner ratio of the developer is excessively high, the toner charge amount becomes low, so that a problem such as toner scattering is generated. On the other hand, when the toner ratio of the developer is excessively low, the carrier is deposited to disturb the toner image developed from the electrostatic latent image or electric charges flow from the carrier to the latent image bearing member to disturb the electrostatic latent image.
Further, even when the toner ratio in the developing device is roughly a desired value, in the case where there is a local non-uniformity of the toner ratio of the developer supplied to the electrostatic latent image bearing member, non-uniformity in amount of the toner used for development is generated depending on the non-uniformity of the toner ratio. Therefore, it is desirable that the ratio of the toner, before the development, conveyed on the developer carrying member is constant to the possible extent.
In order to solve such a problem, as in Japanese Patent No. 3127594, a developing device constitution such that the toner ratio of the developer supplied to the latent image bearing member is constant has been proposed.
In order to explain the developing device constitution in Japanese Patent No. 3127594, from the viewpoint such that the toner ratio of the developer on the developer carrying member is made constant to the possible extent, the constitution of the two-component developing device is roughly classified into a conventional type developing device and a new type developing device. These two constitutions are different in positional relation between a developer circulation path and the developer carrying member.
In general, in the developing device, the developer carrying member for conveying the developer toward the latent image bearing member and the developing container in which a feeding screw for circulating the developer in the developing container is incorporated are provided. When the developing container is considered in a function-separation manner, the developing container includes a developer feeding chamber for feeding the developer to the developer carrying member and a collecting chamber for collecting, in the developing container, the developer lowered in toner ratio by conveying the developer to an opposing portion in the neighborhood of the latent image bearing member by the developer carrying member to develop the electrostatic latent image into the toner image. Further, the developing container includes a developer stirring chamber for stirring a supply toner in an amount depending on the amount of the collected developer and the amount of the toner used for development to make the toner ratio in the developing container substantially constant.
Further, the developing container includes a first connecting portion for connecting a downstream portion of the developer stirring chamber with respect to a developer conveying (feeding) direction and an upstream portion of the developer feeding chamber with respect to the developer conveying direction. Further, the developing container includes a second connecting portion for connecting a downstream portion of the developer feeding chamber with respect to the developer carrying direction and an upstream portion of the developer stirring chamber with respect to the developer conveying direction. Further, the developing container includes a supplying portion for supplying the toner depending on the amount of the consumed toner. In general, in order to make the toner ratio in the developer feeding chamber substantially constant, a toner supplying opening is provided in the neighborhood of the second connecting portion between the downstream portion of the developer feeding chamber with respect to the developer conveying direction and the upstream portion of the developer stirring chamber with respect to the developer conveying direction.
In the conventional type developing device, the developer is fed to the developer carrying member from the developer feeding chamber having a feeding function and a collecting function, and the developer used for development is collected. For this reason, the developer used for development and lowered in toner ratio is included in the developer stirring chamber, and therefore the local non-uniformity of the toner ratio is liable to occur. Further, the toner ratio is more lowered with a position close to a downstream end of the developer feeding chamber.
On the other hand, in the new type developing device, the feeding function and the collecting function are independent of each other. For this reason, when the toner ratio of the developer entering the developer feeding chamber is constant, the toner ratio is constant in the entire region of the developer feeding chamber.
However, in the above-described developing device constitution (the new type developing device), the developer is not collected in the developer feeding chamber and therefore there is a tendency that the amount of the developer in the developer feeding chamber is not readily maintained. When the developer amount in the developer feeding chamber is insufficient, the developer cannot be fed onto the developer carrying member and therefore there arises a problem such that a part of the electrostatic latent image for an image cannot be developed.